feat(mathjax): upgrade mathjax from v2 to v4

[navinkarkera]

Summary

Upgrade MathJax from v2.7.5 → v4.1.2 across all XBlocks (problem, discussion, html). The v2 MathJax.Hub.Queue(...) / .Hub.Config() / .Hub.getAllJax() API is removed upstream. This PR ports all JS interactions to v4's MathJax.typesetPromise() / MathJax.typesetClear() / MathJax.startup.promise API.

Co-authored by: GPT 5.5(xhigh)

Manual testing

Before merging, verify each xblock type in a devstack with the corresponding edx-platform MathJax v4 branch:

• Checkout feat!: upgrade to mathjax v4 openedx-platform#33555 in openedx-platform.

1. Problem XBlock — formula preview (AsciiMath / calculator syntax)

1. Create a problem containing <formulaequationinput> (e.g., the built-in "Formula Problem" template).
2. Type R_1*R_2/R_3 into the input field.
3. Expected: The preview below the input renders the formula (no raw text). The loading spinner <img.loading> remains visible while a request is in flight and is not destroyed by the preview update.
4. Open devtools console. Type sin(x) and sqrt(y^2+1).
5. Expected: No console errors. Each keystroke updates the rendered preview.

2. Problem XBlock — formula preview (explicit TeX syntax)

1. In the same input, type \(E=mc^2\).
2. Expected: The preview renders E=mc² as math; the TeX delimiters are preserved and not double-wrapped in backticks.
3. Type $$\sum_{i=1}^{\infty} \frac{1}{n^2}$$.
4. Expected: Display math renders correctly with double-dollar delimiters preserved.

3. Discussion — math rendering in posts

1. Navigate to a course discussion topic.
2. Create a post containing $x^2 + y^2 = z^2$ (inline) and $$\int_0^\infty e^{-x}\,dx$$ (display).
3. Expected: Both render as math, not raw source. The discussion thread view, profile view, and response comments all show typeset math.
4. Edit an existing post that contains math. The WMD editor should not throw errors related to MathJax.Hub.Browser.
5. Expected: Editing works, the IE branch (if applicable) falls back to UA sniffing without throwing.

4. HTML XBlock — math in content

1. Create an HTML component containing \[ \sum_{i=1}^n i = \frac{n(n+1)}{2} \].
2. Save and view the unit.
3. Expected: Display math renders. The HTML XBlock calls MathJax.startup.promise.then(() => typesetPromise([element])) on initialization — no console errors.

5. Graceful degradation — CDN failure

1. Open devtools → Network tab → block cdn.jsdelivr.net.
2. Reload any page with math content.
3. Expected: Page loads without JS errors. No unhandled promise rejections from startup.promise.then(...) chains (.catch() is present in formula_equation_preview.js; utils.js and html.py skip typesetting when MathJax is absent).

Risks

• Host dependency: xblocks-contrib expects the host platform (edx-platform) to provide MathJax v4 with input/asciimath loaded. Its own mathjax_include.js is a guarded fallback (if (typeof MathJax === 'undefined')). If the host changes config without enabling AsciiMath input, formula preview (backtick-delimited syntax) breaks silently.
• noerrors package: Loaded in config; silently swallows MathJax rendering errors. Same behavior as v2, intentional.
• messageStyle: "none" not ported — v4 may show brief "Loading MathJax..." overlay. Cosmetic only.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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